616 research outputs found
The puzzle of the synthesis of the rare nuclide 138La
The calculations of the p-process in the O/Ne layers of Type II supernovae
are quite successful in reproducung the solar system content of p-nuclides.
They predict, however, a significant underproduction of the rare odd-odd
nuclide 138La. A model for the explosion of a 25 Mo star with solar metallicity
is used to suggest that electron neutrino captures on 138Ba may well be its
most efficient production mechanism. The responsibility of an inadequate
prediction of the 138La and 139La photodisintegration rates in the too low
production of 138La is also examined quantitatively. A detailed discussion of
the theoretical uncertainties in these rates suggest that the required rate
changes are probably too high to be fully plausible. Their measurement would be
most welcome. They would help disentangling the relative contributions of
thermonuclear and neutrino processes to the 138La production.Comment: 4 pages to be published by A&A Letter
Reaction rate uncertainties and 26Al in AGB silicon carbide stardust
Stardust is a class of presolar grains each of which presents an ideally
uncontaminated stellar sample. Mainstream silicon carbide (SiC) stardust formed
in the extended envelopes of carbon-rich asymptotic giant branch (AGB) stars
and incorporated the radioactive nucleus 26Al as a trace element. The aim of
this paper is to analyse in detail the effect of nuclear uncertainties, in
particular the large uncertainties of up to four orders of magnitude related to
the 26Al_g+(p,gamma)27Si reaction rate, on the production of 26Al in AGB stars
and compare model predictions to data obtained from laboratory analysis of SiC
stardust grains. Stellar uncertainties are also briefly discussed. We use a
detailed nucleosynthesis postprocessing code to calculate the 26Al/27Al ratios
at the surface of AGB stars of different masses (M = 1.75, 3, and 5 M_sun) and
metallicities (Z = 0.02, 0.012, and 0.008). For the lower limit and recommended
value of the 26Al_g(p,gamma)27Si reaction rate, the predicted 26Al/27Al ratios
replicate the upper values of the range of the 26Al/27Al ratios measured in SiC
grains. For the upper limit of the 26Al_g(p,gamma)27Si reaction rate, instead,
the predicted 26Al/27Al ratios are approximately 100 times lower and lie below
the range observed in SiC grains. When considering models of different masses
and metallicities, the spread of more than an order of magnitude in the
26Al/27Al ratios measured in stellar SiC grains is not reproduced. We propose
two scenarios to explain the spread of the 26Al/27Al ratios observed in
mainstream SiC, depending on the choice of the 26Al_g+p reaction rate. One
involves different times of stardust formation, the other involves extra-mixing
processes. Stronger conclusions will be possible after more information is
available from future nuclear experiments on the 26Al_g+p reaction.Comment: 6 pages, 5 Postscript figures, accepted for publication in Astronomy
and Astrophysic
The Karlsruhe Astrophysical Database of Nucleosynthesis in Stars Project - Status and Prospects
The KADoNiS (Karlsruhe Astrophysical Database of Nucleosynthesis in Stars) project is an astrophysical online database for cross sections relevant for nucleosynthesis in the s process and the γ process. The s-process database (http://www.kadonis.org) was started in 2005 and is presently facing its 4th update (KADoNiS v1.0). The γ-process database (KADoNiS-p, http://www.kadonis.org/pprocess) was recently revised and re-launched in March 2013. Both databases are compilations for experimental cross sections with relevance to heavy ion nucleosynthesis. For the s process recommended Maxwellian averaged cross sections for kT=5-100 keV are given for more than 360 isotopes between 1H and 210Bi. For the γ-process database all available experimental data from (p, γ), (p, n), (p, α), (α, γ), (α, n), and (α, p) reactions between 70Ge and 209Bi in or close to the respective Gamow window were collected and can be compared to theoretical predictions. The aim of both databases is a quick and user-friendly access to the available data in the astrophysically relevant energy regions. © 2014 Elsevier Inc.Peer reviewe
Vibrational spectroscopy coupled to a multivariate analysis tiered approach for argentinean honey provenance confirmation
In the present work, the provenance discrimination of Argentinian honeys was used as case study to compare the capabilities of three spectroscopic techniques as fast screening platforms for honey authentication purposes. Multifloral honeys were collected among three main honey-producing regions of Argentina over four harvesting seasons. Each sample was fingerprinted by FT-MIR, NIR and FT-Raman spectroscopy. The spectroscopic platforms were compared on the basis of the classification performance achieved under a supervised chemometric approach. Furthermore, low- mid- and high-level data fusion were attempted in order to enhance the classification results. Finally, the best-performing solution underwent to SIMCA modelling with the purpose of reproducing a food authentication scenario. All the developed classification models underwent to a “year-by-year” validation strategy, enabling a sound assessment of their long-term robustness and excluding any issue of model overfitting. Excellent classification scores were achieved by all the technologies and nearly perfect classification was provided by FT-MIR. All the data fusion strategies provided satisfying outcomes, with the mid- and high-level approaches outperforming the low-level data fusion. However, no significant advantage over the FT-MIR alone was obtained. SIMCA modelling of FT-MIR data produced highly sensitive and specific models and an overall prediction ability improvement was achieved when more harvesting seasons were used for the model calibration (86.7% sensitivity and 91.1% specificity). The results obtained in the present work suggested the major potential of FT-MIR for fingerprinting-based honey authentication and demonstrated that accuracy levels that may be commercially useful can be reached. On the other hand, the combination of multiple vibrational spectroscopic fingerprints represents a choice that should be carefully evaluated from a cost/benefit standpoint within the industrial context
Proton capture cross section of Sr isotopes and their importance for nucleosynthesis of proton-rich nuclides
The (p,) cross sections of three stable Sr isotopes have been
measured in the astrophysically relevant energy range. These reactions are
important for the -process in stellar nucleosynthesis and, in addition, the
reaction cross sections in the mass region up to 100 are also of importance
concerning the -process associated with explosive hydrogen and helium
burning. It is speculated that this -process could be responsible for a
certain amount of -nuclei in this mass region. The (p,) cross
sections of Sr isotopes were determined using an activation
technique. The measurements were carried out at the 5 MV Van de Graaff
accelerator of the ATOMKI, Debrecen. The resulting cross sections are compared
with the predictions of statistical model calculations. The predictions are in
good agreement with the experimental results for Sr(p,)Y
whereas the other two reactions exhibit differences that increase with mass
number. The corresponding astrophysical reaction rates have also been computed.Comment: Phys. Rev. C in pres
Neutron-induced astrophysical reaction rates for translead nuclei
Neutron-induced reaction rates, including fission, are calculated in the
temperature range 1.d8 <T (K) < 1.d10 within the framework of the statistical
model for targets with atomic number 83 < Z < 119 (from Po to Uuo) from the
neutron to the proton drip-line. Four sets of rates have been calculated,
utilizing - where possible - consistent nuclear data for neutron separation
energies and fission barriers from Thomas-Fermi (TF), Extended Thomas-Fermi
plus Strutinsky Integral (ETFSI), Finite-Range Droplet Model (FRDM) and
Hartree-Fock-Bogolyubov (HFB) predictions. Tables of calculated values as well
as analytic seven parameter fits in the standard REACLIB format are supplied.
We also discuss the sensitivity of the rates to the input, aiming at a better
understanding of the uncertainties introduced by the nuclear input.Comment: 14 pages, 10 figures, 2 tables in paper, 2 in Annex and online tables
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On the Dynamics of Proto-Neutron Star Winds and r-Process Nucleosynthesis
We study here the formation of heavy r-process nuclei in the high-entropy
environment of rapidly expanding neutrino-driven winds from compact objects. In
particular, we explore the sensitivity of the element creation in the A>130
region to the low-temperature behavior of the outflows. For this purpose we
employ a simplified model of the dynamics and thermodynamical evolution for
radiation dominated, adiabatic outflows. It consists of a first stage of fast,
exponential cooling, followed by a second phase of slower evolution, either
assuming constant density and temperature or a power-law decay of these
quantities. These cases are supposed to capture the most relevant effects of a
strong deceleration or decreasing acceleration of the transsonic outflows,
respectively, e.g. in a wind termination shock caused by the collision with the
slower, preceding supernova ejecta. We find that not only the transition
temperature between the two expansion phases can make a big difference in the
formation of the platinum peak, but also the detailed cooling law during the
later phase. Unless the transition temperature and corresponding (free neutron)
density become too small (T < 2*10^8 K), a lower temperature or faster
temperature decline during this phase allow for a stronger appearance of the
third abundance peak. Since the nuclear photodisintegration rates between
~2*10^8 K and ~10^9 K are more sensitive to the temperature than the n-capture
rates are to the free neutron density, a faster cooling in this temperature
regime shifts the r-process path closer to the n-drip line. With low (gamma,n)-
but high beta-decay rates, the r-processing then does not proceed through a
(gamma,n)-(n,gamma) equilibrium but through a quasi-equilibrium of
(n,gamma)-reactions and beta-decays, as recently also pointed out by Wanajo.Comment: 18 pages, 14 figures with 25 eps plots; referee comments included;
accepted by Astronomy & Astrophysic
Explosive Nucleosynthesis: What we learned and what we still do not understand
This review touches on historical aspects, going back to the early days of
nuclear astrophysics, initiated by BFH and Cameron, discusses (i) the
required nuclear input from reaction rates and decay properties up to the
nuclear equation of state, continues (ii) with the tools to perform
nucleosynthesis calculations and (iii) early parametrized nucleosynthesis
studies, before (iv) reliable stellar models became available for the late
stages of stellar evolution. It passes then through (v) explosive environments
from core-collapse supernovae to explosive events in binary systems (including
type Ia supernovae and compact binary mergers), and finally (vi) discusses the
role of all these nucleosynthesis production sites in the evolution of
galaxies. The focus is put on the comparison of early ideas and present, very
recent, understanding.Comment: 11 pages, to appear in Springer Proceedings in Physics (Proc. of
Intl. Conf. "Nuclei in the Cosmos XV", LNGS Assergi, Italy, June 2018
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